Recently, as semiconductor lasers for emitting blue violet laser light have been put into practical use, a Blu-ray Disc (hereinafter, referred to as the “BD”), which is an optical information recording medium (hereinafter, referred to also as the “optical disc”) having a higher density and a larger capacity than CDs (Compact Discs) and DVDs (Digital Versatile Discs) while having the same disc diameter therewith, has been put into practical use. A BD is an optical disc having a protective substrate thickness of about 0.1 mm which is used for performing information recording and reproduction with a blue violet laser light source for a wavelength of about 400 nm and an objective lens having a numerical aperture (NA) improved to 0.85. The “protective substrate thickness” means a thickness of a transparent layer (protective substrate) existing between a face of the optical disc on the laser light incidence side and the information recording layer.
An HD DVD having a protective substrate thickness of about 0.6 mm, which is also used with a blue violet laser light source for a wavelength of about 400 nm and an objective lens having a numerical aperture of 0.65, also has been put into practical use. These optical discs usable for performing recording or reproduction with a blue violet laser light source are collectively referred to as the “high density optical discs”.
Under the circumstances, an optical head compatible for performing information recording and reproduction by collecting laser light of different wavelengths to an information recording layer of the respective optical discs having different protective substrate thicknesses using one objective lens is now proposed. In order to realize an optical head having such compatibility, various structures have been devised.
For example, Patent Document 1 discloses an optical head for emitting laser light of three wavelengths of about 405 nm, about 655 nm and about 785 nm. An objective lens mounted on this optical head has high optical performances. Specifically, this objective lens has a diffraction limited performance for optical discs having different protective substrate thicknesses.
FIG. 25 shows a structure of an objective lens 101 described in Patent Document 1. The objective lens 101 includes a reference lens 101a formed of a glass material and a close contact lens 101b formed of a resin material. The close contact lens 101b is formed of a material different from that of the reference lens 101a, and is cemented to or bonded to a face of the reference lens 101a on the optical disc side, in a close contact or substantially close contact manner. Patent Document 1 describes that such an objective lens 101 realizes recording to or reproduction from a high density optical disc, DVD and CD using blue violet laser light.
The objective lens utilizes the difference in the refractive index and dispersion between the reference lens and the close contact lens, and compensates for the aberration for two or three different wavelengths only by the refractive power of the lens. Therefore, as opposed to an objective lens using diffraction, the above-described objective lens has a feature of losing substantially no light amount and thus providing a high light utilization efficiency for all the wavelengths.
By contrast, an optical head having an objective lens which does not use a reference lens or a close contact lens is also known. Specifically, this optical head has diffraction grating on a lens face of one objective lens. This optical head utilizes the diffraction of light to collect laser light of three wavelengths to an information recording layer of the respective optical discs having different protection substrate thicknesses.
As a modification of the above-described optical head, an optical head including a coupling lens having a diffraction function between a light source and an objective lens.
Patent Document 1: International Publication No. 2004/053557 pamphlet